1. Field of the Invention
The present invention relates to a display monitor which is intended to alleviate the fatigue of eyes of the operator.
2. Description of the Prior Art
FIG. 1 is a block diagram showing the principal portions of a conventional display monitor. In the figure, reference numeral 1 denotes an input terminal for receiving a video signal, 2 shows a video signal controller which implements the clamping and amplitude control for the input video signal, and 3 shows a variable resistor which is used to set the intensity of display and produces an intensity control signal to be supplied to the video signal controller 2 for controlling the amplitude of video signal.
A numeral 4 shows an amplifier which amplifies the video signal after it has been processed by the video signal controller, 5 shows variable resistor which is used to adjust the brightness of display and produces a level control signal to be supplied to the amplifier 4 for controlling the d.c. level of the amplified video signal, and 6 shows a cathode ray tube (will be termed "CRT" hereinafter) which displays on its screen a picture which is derived from the amplified video signal.
Next, the operation of the foregoing arrangement will be explained. FIG. 2 is an explanatory diagram showing the signal waveforms at the input and output of the video signal controller 2. The video signal shown by (A) in FIG. 2 received on the input terminal 1 is sent to the video signal controller 2, by which the signal level is clamped and the peak value is rendered the amplification or attenuation from "a" to "b" in accordance with the intensity control signal produced by the intensity setting variable resistor 3, and the resulting video signal shown by (B) in FIG. 2 is sent to the amplifier 4.
The amplifier 4 amplifies the video signal provided by the video signal controller 2 and applies the output to the cathode of the CRT 6, and the picture carried by the video signal is displayed on the CRT screen. The amplified video signal produced by the amplifier 4 has its d.c. level controlled by the level control signal produced by the brightness control variable resistor 5.
The intensity setting variable resistor 3 produces the intensity control signal based on its resistance set by the operator and sends it to the video signal controller 2, which responds to the signal to vary the amplification factor for the video signal so that the intensity of a picture, i.e., characters and figures, displayed on the screen of CRT 6 is adjusted.
The brightness control variable resistor 5 produces a brightness control signal based on its resistance set by the operator and sends it to the amplifier 4, which responds to the signal to vary the d.c. level of the amplified video signal so that the brightness of the whole picture including black portions displayed on the screen of CRT 6 is adjusted.
Another conventional display monitor has a function of adjusting the intensity automatically depending on the display condition. For example, when the display area on the screen is large, the cathode current of the CRT is limited so as to suppress the intensity of the whole area and at the same time prevent the collapse of display, or when the display area is small, the intensity is increased so as to produce a high intensity picture.
Still another conventional display monitor disclosed in Japanese Patent Unexamined Publication No. 3-131181 is designed to control the luminance signal directly so as to improve the balance of intensity of the whole picture.
These conventional display monitors are intended to improve the intensity level of the whole picture in a long period of time, i.e., one frame or more, thereby to prevent the collapse of display.
Still another conventional display monitor disclosed in Japanese Patent Unexamined Publication No. 62-1381 is designed to control the intensity of display depending on the environment (brightness of the room) so as to alleviate fatigue of the eyes of the operator, and this technique is also based on the control of intensity of the whole picture.
Still another conventional display monitor disclosed in Japanese Patent Unexamined Publication No. 2-278284 is designed to adjust the intensity while suppressing the flicker caused by the power noise by selecting a portion of well-balanced discharge luminescence frequencies in every horizontal scanning period, and this technique also controls the intensity within a period of the horizontal sync signal.
Although the conventional display monitors arranged as described above are designed to allow the operator to adjust the intensity of the CRT screen through the use of the intensity setting variable resistor 3, it is fixed at a set level during the operation. The CRT screen has a flicker at the vertical sync frequency as the scanning position moves from top to bottom, causing fatigue of the eyes of the operator. Raising the scanning frequency for suppressing the flicker results in a significant increase of cost.
Any of the foregoing conventional display monitors operates at a constant intensity unless there is a change in the environment (environmental changes include a change in the brightness of room, a change in the displayed picture and a change in the setting of the intensity setting variable resistor), and the CRT screen which is displaying a still picture has a flicker at a constant vertical sync frequency (e.g., 60 Hz) which causes fatigue of the eyes of the operator.